Radiation sources for electromagnetic waves have been increasing in number with continued advances in industrial technology and the general standard of living. The leakage of electromagnetic waves, therefore, has become a significant social problem since the wave can exert adverse influences on the human body. Such leakage may also cause the incorrect behavior of integrated circuits in electronic equipment. In particular, electromagnetic waves radiated from electronic computers and various office equipment exert adverse influences on television sets and audio equipment.
Even in the car industry, electronic devices are now used. For example, they are used in automatic controllers for various machines such as an engine, and further, in a speedometer, a tachometer, and so forth. Several types of cars are now equipped with a microcomputer. In addition, electronic devices such as a telephone, a radio, and a TV set are installed inside cars for the convenience of the driver. However, these electronic devices are liable to suffer from various operational difficulties under the influence of electromagnetic waves emanating from the engine compartment, or from a source outside the car.
Various methods have recently been used to shield such equipment against electromagnetic waves, as described in British Pat. Nos. 1,073,567, 1,168,701, 1,195,076, 1,226,181 and 1,246,829, U.S. Pat. Nos. 3,468,753, 3,491,056, 3,393,257, 3,474,067, 3,668,281, 3,767,613, 4,006,114 and 4,234,469 and French Pat. No. 2,077,021.
In general, since metals can absorb or reflect electromagnetic waves, they are effectively used as shielding materials for electromagnetic waves radiated from an electronic oven and various communication apparatuses. In addition, plastics with metals deposited thereon by vacuum metallizing method or arc spray coating method can be used for the same purpose. Furthermore, materials prepared by adding additives, e.g., carbon powder and metal powder, to plastics can be used.
Using metals alone or other materials coated with metals are disadvantageous in that the specific gravity is large, workability is poor, the procedure of treatment is not simple, and the treatment cost is high. Similarly, a method of introducing additives is disadvantageous in that when these additives are added in small amounts, the effect cannot be exhibited sufficiently. However, when they are added in large amounts, the mechanical strength and workability of the resulting molded article are seriously reduced although the effect can be exhibited.
Plastic molds are provided with a flash sprayed metal layer or an electrically conductive coating layer on the inner surface thereof for the purpose of shielding electromagnetic waves. The flash sprayed metal layer or the electrically conductive coating layer is separated from the plastic mold by the impulsive action of falling bodies. The thus-separated small pieces fall in the plastic mold, causing problems such as the break-down of electric circuits contained in the plastic mold and fire. Furthermore, containers for electronic devices, such as office machines must be flame resistance due to safety requirements. Accordingly, there is a need for a resin composition which is capable of providing a surface layer which is free from the separation and break-down caused by impulses. Further, the material must be flame resistance, have excellent workability, as well as electromagnetic wave-shielding ability.
In general, in order to increase the electromagnetic interference property of synthetic resins or rubbers, it is necessary to incorporate a large amount of electrically conductive carbon black in the resin. However, it is very difficult to provide flame resistance to compositions containing large amounts of carbon black. Even though additives such as halogen-containing organic compounds and antimony oxide are added to provide flame resistance, when the resulting composition is exposed to a flame in accordance with a flame resistance testing method ("UL-94 Method", Underwriter Laboratories, U.S.A.), glowing due to carbonization of the carbon black is vigorous. Furthermore, the flame remains alive for a long period of time, and if other flammable substances are brought into contact with the flame, they will easily catch fire. Even if considerable amounts of flame retarders are added this tendency remains and it is quite difficult to eliminate these defects. Moreover, the addition of such large amounts of flame retarders to compositions with carbon black incorporated thereinto reduces the electromagnetic interference properties since it results in a reduction in the proportion of carbon black, even if flame resistance can be provided. Therefore, it is not possible to obtain a polyolefin composition with good electromagnetic interference properties which also has excellent flame resistance. Furthermore, the use of such large amounts of expensive flame retarders gives rise increased costs. In addition, a molded article prepared from the composition does not have mechanical characteristics which are suitable for practical use.